by adding P+D repetitive control

PLL speed control system has excellent 'macro­ scopic', or long-term, steadiness. However, 'microscopic' speed fluctuation is another important issue. It is called 'cogging,' caused by electro-magnetic structural property of motor. Al­ though repetitive control is useful for reducing such fluctuation, stability condition is hard to be satisfied for conventional PLL speed control systems. In this article, we propose a P+D repetitive controller to ease this condition. Rather simple implementation with an FPGA and effective experimental results revealing triple reduction in amplitude are shown.